1. Field of the Invention
The present invention is directed to a process for extracting hydrocarbon gases from suboceanic marine sediment hydrates. In one embodiment, hydrate rich sediments are drilled and then electrically heated to release hydrocarbon gases with subsequent capture in an overhead receiver. The overhead receiver is raised to a sea depth to permit dissociation of the hydrocarbon gases which are then gathered. In another embodiment, subsea marine sediments containing hydrates are partitioned, and loaded into a container. The container is covered with an overhead receiver and is raised to a shallower sea depth wherein lower pressure and higher temperature permit hydrocarbon gas to be released and gathered.
2. Prior Art
The present invention provides a process to harvest gas hydrates from marine sediments located between the sea floor and the hydrate base line in order to extract hydrocarbon gases.
Gas hydrates are ice-like crystalline solids formed from a mixture of water, methane, and other hydrocarbon gases. They can occur in the pore spaces of marine sediments and can form cements, nodes or layers.
It is known that seawater (hydrothermal) temperature decreases from the sea surface to the sea floor. It is also known that the earth temperature increases beneath the sea floor due to the local geothermal gradient. On the other hand, the hydrate formation temperature (“HFT”) corresponding to the phase boundary increases due to the increase of the hydrostatic pressure.
FIG. 1 illustrates an example of a known subsea temperature profile with depth below the surface of the sea charted against the temperature in ° K. The dashed line having reference numeral 10 illustrates a temperature of 273° K or 0° C., the freezing temperature of fresh water. The dashed line with reference numeral 12 illustrates an example of the temperature of sea water at different subsurface depths. The straight line having reference numeral 14 illustrates an example of the depth of the sea water with sea water above and sediment below. The parallel dashed line 16 illustrates the base of the hydrate depth. Below the dashed line 16, it is generally too warm for solid hydrates to form. It can be seen that the temperature is the lowest at the sea floor. The arched line 18 illustrates the phase boundary between hydrate solids and water/gases with hydrate solids to the left and water and gases to the right of line 18.
As illustrated by dashed line 12, below the sea floor, the temperature rises again with a certain geothermal gradient. When the water depths exceed 300 to 500 meters, gas hydrates are stable in a zone from the sea floor to the hydrate baseline where the temperature is equal to the HFT. This zone is called the Hydrate Stability Zone (HSZ). The heat needed to melt the hydrates in the region close to the hydrate baseline is relatively small.
According to the US Geological Survey, gas hydrates bind immense amounts of methane and other hydrocarbon gases in sea-floor sediments. Natural conditions exist suitable for the formation of hydrocarbon bearing hydrates in a subsea layer covering much of the earth. If produced cost effectively, they could serve as a stable energy supply. At least three methods have been proposed in the past for hydrocarbon gas production from hydrates, including thermal injection, depressurization, and hydrate inhibitor injection.
Examples of prior art proposals include:
Elliot et al. (U.S. Pat. No. 4,376,462) which discloses pumping relatively warm brine water down to hydrates in the sea bed through a conduit, allowing the brine to circulate through the hydrates to melt and produce gaseous hydrocarbons, and then separating gaseous hydrocarbons from the spent brine.
Satoru et al. (Japanese Publication No. JP2005139825) which discloses in an abstract the use of warm water which is fed to hydrates to gasify and a subsequent recovery mechanism.
Russian Patent Abstract (SU1792482) which discloses a drilling rig lowered to the sea bed with a drilling tube which is connected to a heated drum 8. A dome-shaped folded element is opened, the hydrates are partially decomposed and then transferred by internal pressure into a heated drum for further processing.
Michihiro et a. (Japanese Publication No. 2004204562) discloses in an abstract a subsea boring device 11 to drill a plurality of horizontal wells 30 into a gas hydrate layer 2. Warm heat is sent into the layer 2 in order to decompose the gas hydrates.
The Pfefferie references (U.S. Pat. No. 6,973,968, U.S. Patent Publication No. 2005/0016725 and U.S. Patent Publication No. 2005/0284628) disclose injecting combination products containing carbon dioxide into a hydrate deposit into a well drilled in the sea bed for combustion in order to produce a heated fluid.
German PCT Application WO2003/021079 provides an abstract which discloses introducing fluid from the surface which destabilizes gas hydrates to release gases which are drawn off to above the surface of a riser.
Cottle (U.S. Pat. No. 4,007,787) discloses injection of normally liquid light hydrocarbons into a hydrate reservoir in order to disclose hydrates with optional injection of a freezing point depressant.
Chatterji et al. (U.S. Pat. No. 5,713,416) discloses injecting and combining an acidic liquid with a basic liquid to form an exothermic reaction producing a hot salt solution to thermally decompose gas hydrates which are produced out of the formation.
Heinemann et al. (U.S. Pat. No. 5,964,093) discloses a sunlight permeable top for a gas hydrate storage cavity.
Heinemann et al. (U.S. Pat. No. 6,214,175) discloses in FIG. 3 a downhole microwave generator which applies electromagnetic radiation to disassociate hydrates in order to release gases.
Nohmura (U.S. Pat. No. 6,192,691) discloses a flexible sheet 2 which is sunk to the sea floor to trap methane hydrate gas which is filled up by the buoyancy of the gasified methane.
Wyatt (U.S. Pat. No. 6,299,256) discloses a flexible cover 10 with steerable pods 12 with a mining module 14 connected to an inside surface of the cover 10 to dislodge deposits by mechanically agitating and/or heating and thawing.
No viable technologies, however, for extracting gas hydrates from deep ocean deposits have been developed to date.
Due to the shallow depths and low permeability of the HSZ, low productivity is anticipated for normal gas production from gas hydrates in marine sediments. Therefore, only a large number of low cost wells could support an offshore production facility and pipeline transport to shore. A way of harvesting natural gas from sea floor gas hydrates presented in the present invention is a combination of new concepts aimed at overcoming technical barriers, maintaining cost and energy efficiencies, and minimizing safety and environmental concerns.
Accordingly, it is a principal object and purpose of the present invention to provide a process to harvest gas hydrates from marine sediments located between the sea floor and the hydrate base line.
It is a further object and purpose of the invention to provide a method to harvest gas hydrates using the nature of the hydrates and subsea pressure and temperature profiles to provide a simple and open production system which is generally safe, economical, energy efficient and environmentally friendly.